Microemulsions have many advantages, including: (1) high solubilization capacity of both water-soluble and oil-soluble drugs; (2) thermodynamically stable; (3) transparent; and (4) formed spontaneously without addition of external energy. A 1998 review on the state of the art of microemulsion drug delivery indicates that the major advantages of microemulsions over other systems derives from their ability to increase the solubility of poorly soluble drugs (in water) in the vehicle and increase interfacial area for mass transfer (due to small particle size) resulting in an increase in drug permeability through epithelial tissues (Malmstem, Handbook of Microemulsion Science and Technology. Kumar, P. Ed. Marcell Dekker, Inc.: New York, 1999). In the same review, it is indicated that while nonionic surfactants significantly reduce the toxicity associated with microemulsions, fully non-toxic and biocompatible microemulsions would only be possible when microemulsions could be formulated with natural surfactants (e.g., lecithin) and avoiding the use of short and medium chain alcohols.
In a more recent review (Prausnitz, M. R.; Mitragotri, S. and Langer, R. Nat Rev Drug Discovery, 2004, 3(2):115-24) different drug delivery methods, including surfactant systems, have been compared. The authors indicate that surfactant-based vehicles typically increase drug permeability by solubilizing the lipids and denaturing the keratin of the stratum corneum, leading to an increase in toxic side-effects such as skin erythema and eczema. The authors indicate that new breakthroughs in surfactant-based, and other chemical enhancer formulations would be possible only through a better understanding of the vehicle-drug-skin interactions.
U.S. Pat. No. 5,654,337 to Roentsch et al. describes a formulation used for the delivery of pharmaceutically active agents through the skin. It consists of a biocompatible organic solvent (isopropyl myristate), a polar lipid (lecithin), a surfactant, water and urea. This work produces a “speed-gel” product which is similar to organogels (microemulsion-based gel), but differs from microemulsions in that the speed-gel is meta-stable (due to the high viscosity) and microemulsions are thermodynamically stable and can be formulated having low viscosities, making microemulsions suitable for a wider range of applications.
An oil-in-water microemulsion is described in U.S. Pat. No. 5,688,761 to Owen et al. This microemulsion is suitable for administration of water-soluble protein drugs. By the addition of aqueous fluid, it readily converts an oil-in-water emulsion to a water-in-oil microemulsion and releases the active agent. However, it is ineffective in actual application due to a need of surfactant modifiers to lower the surfactant content to the amount stated in the claims.
U.S. Pat. No. 6,191,105 to Ekwuribe et al. describes a water-in-oil microemulsion with a hydrophilic and lipophilic balance (HLB) value between 3 and 7 is described for oral or parenteral delivery of insulin protein covalently coupled with a polymer. In this particular formulation, the uses of polyethylene glycols or propylene glycols (which increase the viscosity of the formulation and are not readily metabolized) are essential to formulate the microemulsion system.
An oil-in-water or bicontinuous microemulsion is described in U.S. Pat. No. 6,602,511 to von Corswant which is suitable for parenteral, oral and transdermal administration of lipophilic drugs. This system consists of a hydrophilic surfactant, a hydrophobic surfactant (lecithin), a surfactant film modifier, and one or more components for adjusting the polarity of the microemulsion. Such system is said to provide a pharmaceutically acceptable non-toxic vehicle. However, the formulations described in this patent contain 3-13% ethanol which may produce tissue irritation and sensitization after repeated dose. Similar to U.S. Pat. No. 6,191,105 this patent also requires the use of hydrophilic polymers such as polyethylene glycol.
Lastly, U.S. Pat. No. 6,638,537 to Dennis et al., describes an oil-in-water microemulsion for intravenous delivery of oil-soluble drugs. This microemulsion consists of a long polymer chain surfactant and a short fatty acid (C8-C12) surfactant as a co-surfactant. The release rate of a drug from the microemulsion micelles are briefly disclosed by comparing with the commercially available drug preparation. The microemulsion does not contain any alcohol and yet it produces satisfactory results. One problem with the formulation, however, is that it requires at least 25% w/v surfactant to form the microemulsion system which increases viscosity costs, and potential toxic side effects.
It has been found that mixtures of lipophilic linkers such as oleyl alcohol (amphihilic molecules with HLB of 5 or less, and more than 9 carbons in the alkyl group), hydrophilic linkers such as hexyl glucoside (amphiphilic molecules with 6 to 9 carbons in the alkyl group), and lecithin produce microemulsions with a wide range of oils and without the need for short or medium chain alcohols (Acosta, E.; Nguyen, T.; Witthayapanyanon, A.; Harwell, J. H. and Sabatini, D. A. Linker-based bio-compatible microemulsions, Enviro. Sci. Technol. 2005, 39: 1275-1282).
The use of linker microemulsions in drug delivery systems and their cytotoxic effects are not known. What are needed therefore are linker-based lecithin formulations for drug delivery systems that increase drug uptake by an increase of drug absorption in epithelial tissue with minimum cytotoxic side effects.